An undercover informant for the FBI passed a note to Ald. Howard Brookins last year that said “12K to you for your letter of support” as he tried to get the alderman’s essential backing for a liquor license in his 21st Ward, a criminal complaint alleges.

The compliant, released Friday, charges Brookins’ chief of staff, Curtis V. Thompson Jr., 62, of the 4700 block of South Greenwood, with taking a $7,500 cash bribe from the informant at a 2013 Christmas party at Brookins’ office. The informant got the letter of support from the alderman’s office for a convenience store selling alcohol, the complaint says.

Brookins has not been charged and is not even referred to by name in the criminal complaint.

But it is clear from details in the court document that he is “Alderman A” — the alderman Thompson works for in Brookins’ 21st ward.

Brookins, the chairman of the City Council’s Black Caucus, was stunned when told Friday of the federal charges against his chief of staff.

“Are you serious? He’s been charged? Unbelievable. It’s just unbelievable. I am speechless at this point,” Brookins said.

Acknowledging that Thompson “does work for me in that ward,” Brookins said it would be “logical” that he is the “Alderman ‘A’” described in the complaint. He noted that, “People come to us all the time,” seeking aldermanic support for a liquor license needed to open a bar, restaurant or packaged goods store.

But Brookins said he knew of no instance in which Thompson either solicited or accepted a bribe in exchange for convincing the alderman to support a liquor license.

“I’ve known Curtis a while. He has been a pretty stand-up guy. He worked in the mortgage banking field,” the alderman said.

Asked if he trusted Thompson, Brookins said, “I had up until I’m hearing this.”

The federal complaint claims that Brookins met with the FBI’s informant and Thompson at his office on Oct. 29, 2013, and was handed a post-it with the bribe offer. The informant was outfitted with hidden audio and video recording devices. When a reporter attempted to question Brookins about those details, he cut the conversation short.

“I need to call a couple of people, including my attorney. Let me look into it and call you back.”

He did not return repeated phone calls after that.

Thompson — who faces up to 10 years in prison if convicted — declined to comment after he was released on bond Friday afternoon following an initial court appearance.

The complaint says a federal investigation began in 2012 but led to an Oct. 9, 2013, meeting at which the informant handed Thompson a note offering the $7,500 bribe in return for a letter of support for his liquor license from the alderman.

“Okay, I understand, I understand.” Thompson allegedly replied, stating that he would bring it up with the alderman.

At a meeting at Brookins’ office three weeks later, Brookins allegedly told the informant that he would get his “teeth kicked in” by residents of his South Side ward if he allowed another liquor store to open.

Brookins advised the informant that other liquor store owners in his ward would also be upset because they all supported him “big time,” the complaint states.

But he allegedly told the informant it might be easier for him to support a liquor license for a “name brand” store such as a White Hen or 7-Eleven.

After the informant allegedly handed Brookins a note with a $12,000 bribe offer, the alderman is accused of saying that he’d like to support him and would get back to him with a decision.

Two days later, Thompson was recorded saying that the informant “needs to quit writing things down,” the complaint states.

The alderman’s office went on to shake down the informant for more money — donations to the alderman’s holiday toy drive and to cosponsor the alderman’s Christmas party, the complaint alleges.

After the informant agreed to pay up, he was handed a letter from the alderman which read, “Please allow this letter to serve as my full support for a 7-Eleven convenience store… This store will be a welcomed addition to [my] community and those that patronize the area for shopping and convenience needs. As well as wine and spirits (alcohol).”

The informant attended the alderman’s Christmas party on Dec. 19, and while walking out of the party with Thompson, allegedly gave Thompson a red envelope containing a Christmas card and 75 $100 bills. Thompson took the envelope from the informant, “felt it deliberately, looked at it and then placed it in a jacket pocket,” the complaint states.

Thompson then allegedly thanked the informant, and told him, “I do all the work with little acknowledgment, so I don’t know how long I’m gonna be around.”